Sickle cell disease and thalassemia are characterized by pathological red cell dehydration. The mean corpuscular hemoglobin concentration of sickle erythrocytes critically determines the lag time preceding the rapid phase of deoxygenation-induced polymerization of hemoglobin S. Several erythroid ion transporters and channels are believed on the basis of pharmacological and physiological studies to regulate red cell hemoglobin concentration secondary to regulation of red cell volume. Among these activities already studied as therapeutic targets in sickle cell disease are the KCNN/IK1/SK4 Ca2+-activated K+ channel of intermediate conductance (Gardos channel), several types of KCC K-CI cotransporters, at least two types of erythroid CI- conductance, and at least one type of Ca2+permeable cation conductance. The K-CI cotransporters have also been tested as therapeutic targets for thalassemia. The genes encoding these ion-transporting polypeptides are strong candidate risk modifier genes for the hemoglobinopathies. This application proposes the general hypothesis that genetic modulation of these transporter and channel activities will modulate disease severity in mouse models of hemoglobinopathies. This general hypothesis will be tested by experiments designed to pursue the following Specific Aims: Aim 1. We will test the hypothesis that genetic deficiency of the erythroid IK1/Gardos channel will decrease pathologic red cell dehydration and will ameliorate clinical severity in mouse models of sickle cell disease. Aim 2. We will test the hypothesis that genetic deficiency of erythroid KCC K-CI cotransporters will decrease pathologic red cell dehydration and will ameliorate clinical severity in mouse models of sickle cell disease and of beta-thalassemia intermedia. Aim 3. We will test the hypothesis that combined genetic deficiency of erythroid IK1/Gardos channel and of erythroid KCC K-CI cotransporters will further ameliorate clinical severity in mouse models of sickle cell disease. The proposed experiments will increase understanding of sickle cell disease and thalassemia by providing mouse models for genetic tests of new drug therapies under development for near-term clinical testing.